For a fuel cell system that generates electrical power through electrochemical reaction between hydrogen-containing fuel gas and oxidizing gas, a technique is widely known that promotes reuse of hydrogen contained in anode offgas by circulating the anode offgas to anode electrode side of a fuel cell, in order to use the hydrogen contained in the anode offgas efficiently for the generation of electrical power. In such hydrogen circulation type of fuel cell system, it is known that nitrogen permeated from cathode electrode side through an electrolyte membrane of the fuel cell, impurities contained in the fuel gas, and the like accumulate on the anode electrode side of the fuel cell and reduce hydrogen partial pressure, thus resulting in decrease in power generating efficiency of the fuel cell.
So, a technique was made public that provides an electrochemical cell that condenses impurities by selectively permeating hydrogen contained in anode offgas in a circulation pathway for circulating anode offgas as described above for the purpose of maintaining power generating efficiency, and discharges the impurities in the anode offgas that were condensed as a result of the hydrogen permeation out of the system (see Patent Document 1, for example). In case of discharging anode offgas out of the system as above, it is important to reduce an amount of hydrogen contained therein as much as possible also from the viewpoint of efficient utilization of hydrogen, and also because of this reason, it is required to measure concentration of hydrogen in the gas more accurately.
As a technique for measuring concentration of hydrogen contained in measurement target gas, the technique described in Patent Document 2 is disclosed here. This technique, in a hydrogen concentration sensor that employs a proton conducting electrolyte membrane, makes an attempt to achieve measurement of hydrogen concentration by keeping diffusion speed of measurement target gas at an entrance electrode lower than proton conducting ability between the entrance electrode and an exit electrode, thereby reducing influence of moisture contained in the measurement target gas.